client: addFile closes the source file before io.Copy — multipart file uploads always fail

In client.go (current master, v1.22.66), addFile closes the opened file before calling io.Copy(part, file):

file, err := os.Open(filepath.Clean(path))
if err != nil {
    return err
}
err = file.Close()   // closes here...
...
_, err = io.Copy(part, file)  // ...then reads from a closed file

Any multipart form-file upload prepared through this helper fails with file already closed.

Repro (Go 1.26):

// addFile(w, "file", tmpfile) always returns:
// read <tmpfile>: file already closed

Fix: close the file only after the copy (defer). I have a fix + regression test ready and will open a PR linked to this issue.

Start in client.go at the addFile helper and reproduce the multipart upload failure described in the issue. Add or run a regression test for a temporary file upload; done means io.Copy can read the source successfully and the upload no longer returns “file already closed”.